Site Editor: Add E2E coverage for view config extensibility

[jorgefilipecosta]

What?

Follow-up to #80319.

Adds end-to-end coverage for extending the Pages view configuration in the Site Editor. The test covers all four configuration mutation methods (merge, remove, set, and replace) and the four UI-facing configuration areas: the default view, available views, layouts, and Quick Edit form.

Why?

The view configuration API did not have a browser test proving that changes made through get_entity_view_config_postType_page travel through the REST API and affect the Site Editor UI.

How?

• Adds a test-only plugin that modifies Core's existing Pages configuration.
• Retitles and filters an existing view, removes the Scheduled view, and adds a new Published after 2020 view with locked status and date filters.
• Configures the default table sorting and fields, restricts layouts to table with a custom Status column width, and changes the Quick Edit form layout and fields.
• Seeds matching and nonmatching pages, navigates the complete Site Editor UI path, and verifies the records shown by the existing and newly added views.

Testing Instructions

1. Start the E2E environment.

2. Run:

   npm run test:e2e -- test/e2e/specs/site-editor/view-config-extensibility.spec.js --project=chromium

The focused Chromium test should pass.

Testing Instructions for Keyboard

Not applicable. This PR only adds automated test coverage and a test-only plugin; it does not change production UI behavior.

Screenshots or screencast

Custom filtered view

The new Published after 2020 view exposes its locked date and status filters and displays only the matching page.

Customized Quick Edit form

The filtered form uses the regular layout and exposes only the Link field.

Use of AI Tools

OpenAI Codex was used to inspect the relevant API and prior E2E review feedback, implement the test coverage, and run the verification steps. The changes were reviewed and refined interactively by the author.

[jorgefilipecosta]

Superseded by #80577. This draft was accidentally opened from a fork; the replacement uses the same commit from a branch hosted directly in WordPress/gutenberg.